In the course of the progress for research and providing development of hydrogen production technique and infrastructure of utilizing hydrogen for realizing the hydrogen energy society, a great demand is expected in the feature for highly pure hydrogen used, for example, in automobile fuel cells, domestic stationary fuel cells, hydrogen stations, and large-scaled chemical plants in the feature, and higher efficiency is demanded for the production thereof.
For the production of hydrogen at present, a method of steam reforming a hydrocarbon fuel at a temperature of about 700° C. (CH4+H2O→CO+3H2) and then performing further CO-shifting at about several hundred ° C. (CO+H2O→CO2+H2) has been utilized generally with a view point of price competitive power. Ingredients of a gas obtained by way of the reactions described above include, in addition to hydrogen, carbon dioxide, carbon monoxide and, further, unreacted hydrocarbons and water. In solid polymer fuel cells which have been started for popularized domestic use in recent years, purification of hydrogen is not performed for lowering the cost, but a gas mixture at a hydrogen concentration of about 60% is supplied as it is to a fuel electrode of a fuel cell, carbon monoxide that poisons the catalyst of the fuel electrode is oxidized into hydrogen dioxide (CO+½O2→CO2) before supply, and removed to a concentration of less than 10 ppm. However, since the fuel cell using the gas mixture has lower power generation efficiency compared with a fuel cell using pure hydrogen, a technique of producing hydrogen at a higher purity in a reduced space and at a low cost has been demanded. Further, in automobile fuel cells, it is necessary to supply hydrogen at 99.99% or higher in addition to the restriction for the CO concentration and a technique of mass producing inexpensive highly pure hydrogen is demanded.
A method of taking out highly pure hydrogen from a hydrogen containing gas mixture includes, for example, an absorption method, a cryogenic separation method, an adsorption method, and a membrane separation method in which the membrane separation method has an advantage that the efficiency is high and the size can be decreased easily. Further, by constituting a membrane reactor having a hydrogen separation membrane inserted therein in a reaction vessel for steam reforming, it is possible to continuously extract hydrogen generated by the reforming reaction from a reaction atmosphere, promote the reforming reaction and the CO shifting reaction simultaneously even at a temperature of about 500° C., thereby producing highly pure hydrogen at high efficiency. Further, in the membrane reactor, expensive noble metal catalysts such as platinum used for CO shifting are not necessary and it is possible to lower the cost and decrease the size of the apparatus. While the purity of a hydrogen gas passing through the hydrogen separation membrane depends on the performance of the hydrogen separation membrane, when it is necessary to further remove CO and improve purification depending on the application use, loads on such steps can be mitigated.
Several hydrogen separation membranes have been proposed under the background of the advantage in the hydrogen production using the hydrogen separation membrane as has been described above. For example, a non-patent document 1 describes a hydrogen separation membrane in which a palladium alloy membrane is supported on a zirconia porous substrate. In the hydrogen separation membrane, since hydrogen is separated by a method of dissolving hydrogen in the form of atoms into a palladium alloy and diffusing them along the concentration gradient thereby allowing only the pure hydrogen to permeate, highly pure hydrogen can be obtained in principle. A non-patent document 2 describes a hydrogen separation membrane in which a silica glass membrane is supported on an alumina-based porous substrate. The hydrogen separation membrane separates hydrogen by the function of a molecular sieve of selectively allowing hydrogen molecules to permeate by utilizing that the silica glass membrane has pores of a size for allowing only the hydrogen molecules to pass therethrough (0.3 nm).